The present invention relates to an inkjet printing method and printing apparatus forming an image directly on a printing medium based on electrostatic inkjet recording with use of an oily ink and being capable of achieving a high print quality and a large printing speed. More specifically, the invention relates to a prevention of the aggregation and/or precipitation of the particles in the oily ink and a redispersion of the ink used for such a method.
As printing methods of forming a print image on a printing medium on the basis of image data signals, the methods based on electrophotography, thermal dye sublimation, thermal melting transfer and inkjet recording are known.
Electrophotography requires processes for forming an electrostatic latent image on a photosensitive drum by charging and exposure, and the system tends to become complicated requiring an expensive apparatus.
In thermal transfer processes, the apparatus is inexpensive, but suffers from a high running cost and the generation of waste as the processes use an ink ribbon.
In contrast, inkjet processes require inexpensive apparatuses and enjoy a low running cost because a direct printing is performed on a printing medium whereby the ink is ejected only onto image areas needed for image formation.
As a method of applying the inkjet technology to printing system, Japanese Patent Laid-Open No. 286939/1998 discloses a printing method comprising adding an inkjet printing apparatus to a rotary press machine, and additionally printing variable numbers or marks on the same printed matters with the inkjet system.
It is further desirable that a printing system can print high-quality image information such as photographic images. Unfortunately, however, with the conventional ink technique that ejects an aqueous or organic solvent-based ink containing dyes or pigments as the colorant by pressure, liquid droplets containing a large amount of solvent are ejected and thus tend to cause blur in the printed image when an expensive dedicated type of paper is not used.
Accordingly, high quality printed images cannot be obtained when ordinary non-dedicated printing stocks or plastic sheets, which are non-absorbent media, are used for printing.
As one of the inkjet techniques, there is known an image-forming method ejecting ink melted and liquefied by applying heat to an ink material that is solid at ambient temperature. By using this type of ink, the blur of the printed image is mitigated, but due to the high ink viscosity during ejection, it is difficult to eject fine droplets, thus the individual printed dot has a large area as well as a large thickness. Accordingly, the formation of high-resolution images is quite difficult.
Furthermore, in image recording by an inkjet process, there take place various problems such as pipe or head choking caused by the precipitation and aggregation of the particulate ingredients in the ink, thus making ink ejection unstable, deteriorating image quality and at the worst terminating ink ejection. In cases where the size of the dispersed particles is large, they tend to sediment when the ink is stationary whereby ink ejection at a constant particle concentration and thus normal image recording become impossible. Furthermore, in some cases, ink ejection completely stops.
Furthermore, after ink-flow is suspended in inkjet recording, aggregates or deposits of the particulate materials in the ink, or foreign matters such as dust sometimes act to choke the ink-flow pipe or the head, thus causing various problems such as unstable ink ejection which leads to image quality deterioration, and at the worst termination of ink ejection. In cases where the size of the dispersed particles is large, they tend to sediment when the ink is stationary whereby ink ejection at a constant particle concentration and thus normal image recording become impossible.
The invention has been devised by taking notice of the above-cited problems; the object of the invention is to provide an inkjet printing method and printing apparatus which can consistently output sharp and crisp prints by an inexpensive and simple process free of developing treatments, and which cope with digital signals.
As a result of eager investigation of the present inventors for solving the above problems, the present invention has been attained by the following means (1) to (21).
(1) Inkjet printing method comprising:
ejecting an oily ink comprising particles to a printing medium with use of an electrostatic field according to image data signals to form an image directly on the printing medium; and
fixing the image to obtain a printed matter,
wherein a prevention of an aggregation and/or a precipitation of the particles is conducted at least during ink circulation, or
an aggregate and/or a deposit of the particles formed at least due to a suspension of ink-flow is redispersed.
(2) The inkjet printing method as described in (1) above, wherein the oily ink comprises:
a nonaqueous solvent having a specific resistance not less than 109 xcexa9cm and a dielectric constant not higher than 3.5 and; and
colored particles dispersed in the nonaqueous solvent.
(3) An inkjet printing apparatus comprising:
an image-forming means for forming an image directly on a printing medium according to image data signals; and
an image-fixing means for fixing the image formed by the image-forming means to produce a printed matter, the image-forming means being an inkjet recording unit comprising a recording head that ejects an oily ink comprising particles with use of an electrostatic field,
wherein at least one aggregation and/or precipitation-preventing means is equipped in an ink-flow channel of the oily ink in an ink circulation, the aggregation and/or precipitation-preventing means being for a prevention of aggregation and/or precipitation of the particles, or
a redispersing means is equipped, the redispersing means being for redispersing of the particles which are in a state of aggregation and/or precipitation formed due to a suspension of ink-flow.
(4) The inkjet printing apparatus as described in (3) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is located just in front of an ink-ejecting part of the recording head.
(5) The inkjet printing apparatus as described in (3) or (4) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means comprises a step selected from agitation, dispersion, mixing and jetting.
(6) The inkjet printing apparatus as described in (5) above, wherein the steps of agitation, dispersion, mixing and jetting are applied individually or in combination.
(7) The inkjet printing apparatus as described in (6) above, wherein the steps of agitation, dispersion, mixing and jetting are applied with a fixed interval, with a non-fixed interval or continuously.
(8) The inkjet printing apparatus as described in any one of (3) to (7) above, wherein at least one of the aggregation and/or precipitation-preventing means and the redispersing means is in the form of a cartridge.
(9) The inkjet printing apparatus as described in any one of (3) to (8) above, wherein the oily ink comprises:
a nonaqueous solvent having a specific resistance not less than 109 xcexa9cm and a dielectric constant not higher than 3.5 and; and
colored particles dispersed in the nonaqueous solvent.
(10) The inkjet printing apparatus as described in any one of (3) to (9) above, which further comprises a dust-removing means that removes dusts present on a surface of the printing medium prior to and/or during printing.
(11) The inkjet printing apparatus as described in any one of (3) to (10) above, wherein the image forming is carried out by moving the printing medium through s rotation of a counter drum arranged in a position facing the recording head with the printing medium interposed between the recording head and the drum.
(12) The inkjet printing apparatus as described in (11) above, wherein the recording head is of a single-channel or multi-channel type and the image forming is carried out by moving the recording head in the direction parallel to the axis of the counter drum.
(13) The inkjet printing apparatus as described in any one of (3) to (12) above, wherein the image forming is carried out by transporting the printing medium inserted between at least a pair of capstan rollers.
(14) The inkjet printing apparatus as described in (13) above, wherein the recording head is of a single-channel or multi-channel type, and the image forming is carried out by moving the recording head along the direction perpendicular to the moving direction of the printing medium.
(15) The inkjet printing apparatus as described in any one of (3) to (14) above, wherein the recording head is of a full-line type having a width substantially equal to that of the printing medium.
(16) The inkjet printing apparatus as described in any one of (3) to (15) above, wherein the inkjet recording unit further comprises an ink-feeding member that feeds the oily ink to the recording head.
(17) The inkjet printing apparatus as described in (16) above, which further comprises an ink-recovery means that gathers the oily ink from the recording head and circulates the oily ink.
(18) The inkjet printing apparatus as described in any one of (3) to (17) above, wherein the inkjet recording unit further comprises an agitating means for agitating the oily ink in an ink tank that stores the oily ink.
(19) The inkjet printing apparatus as described in any one of (3) to (18) above, wherein the inkjet recording unit further comprises a contrlooing means for controlling the temperature of the oily ink kept in a ink tank that stores the oily ink.
(20) The inkjet printing apparatus as described in any one of (3) to (19) above, wherein the inkjet recording unit further comprises an ink concentration-controlling means that controls the concentration of the oily ink.
(21) The inkjet printing apparatus as described in any one of (3) to (20) above, which further comprises a cleaning means that cleans the recording head.